The grant to which this application is a supplement supports a broad program of epidemiologic studies to elucidate the etiology of cancer and identify means of prevention. Through this supplement we propose to strengthen this program by addition of a unit concerned with experimental carcinogenesis and by surveillance of a cohort of women whose use of oral contraceptives is known. Experimental studies will initially be concerned with the role of estrogens in the etiology of cancer of the breast. In the oral contraceptive cohort, frequency of breast disease (benign or malignant), incidence of all cancer and all deaths will be monitored.